So this is to let you know, to remove this package in order to avoid
further problems :)

packages really ought to build if that package is installed…

Should these kind of bugs (FTBFS or producing broken packages in
non-clean environment) be considered RC?

There are other packages, besides install-info, that are known to be
disruptive for build process:
- graphicsmagick-libmagick-dev-compat is very likely to break packages
linking to imagemagick;
- pythonX.Y-minimal can break packages that autodetects existence of
Python.